Feedback on good_lp #70

Hello @jcavat !
Sorry for polluting the issue section, but I finished a first iteration of the improved API for linear programming I was mentioning in #66 (comment). It is in early stage, but I would love to get your feedback on the API design.

Here it is:
- https://github.com/lovasoa/good_lp
- https://crates.io/crates/good_lp
- https://docs.rs/good_lp